If you are reluctant to actually go to Court, you are reliant on GWR being reasonable in their negotiations. A couple of previous threads have suggested that their investigators/staff are willing to negotiate on factual matters (e.g. which journeys were made, or made by you) but are generally unwilling to negotiate on legal matters (e.g. whether the Anytime fare is lawfully due even if disproportionate to the original fare paid).
If you cannot produce the Railcard itself then a witness statement from the person who you are saying travelled, or their parent/guardian, will be better than nothing. I don't think it is much use saying that you don't want to throw them under the bus - you went to the effort of buying them the ticket, so I think the least they can do is to help you out here when you are facing a potential 4-figure liability.
You can send them any evidence you like, the question is whether they will accept it and reduce the amount they're asking for. As above, our (limited) experience of these civil claims is that factual matters can sometimes be conceded by GWR if there is sufficient evidence that the journey wan't made. So it is an approach that might be worth taking.
I would refer to
this email I drafted up for someone in a similar case. Obviously not all of the same arguments will apply to your case, but it is worth double checking that the Anytime fares demanded are actually correct (i.e. they aren't numerically wrong or overcharging you when a cheaper route of fare was applicable).
If that 'sanity check' passes then it leaves the issues of:
- The lack of legal basis for recovering an admin fee. NB - the fact that the industry is changing the NRCoT later this month to add in a clause, condition 9.3, which (purports to) give them this right suggests that they know they don't have a legal basis for admin fees at the moment.
- The lack of legal basis for recovering fares on behalf of other TOCs, in the absence of evidence of authority to do so. This covers the majority of the journeys made, and a substantial proportion of the £1100 figure they claim you owe, so it is not a point I would give way on readily.
And then arguments about the applicability and enforceability of NRCoT condition 9.2.1 under contract and consumer law.